Project Summary/Abstract The etiology, mechanism and progression of Alzheimer's Disease (AD), and the relationship of AD pathology to clinical manifestations, are not fully understood. Emerging studies suggest that inflammation and microglial activation is an important contributor to AD pathogenesis and progression. The association between tau and microglia is especially critical, since tau is most closely associated with AD. The conventionally accepted sequence of this interaction is that misfolded tau, which is pro-inflammatory, causes microglial activation, leading to dendritic pruning and eventually neuronal cell death. However, it is recently emerging that microglial activation can itself cause tau aggregation and subsequent propagation. Therefore the causality of these interactions is controversial, and requires much needed elucidation in humans in vivo. The goal of this proposal is to understand the interaction and causal sequencing between tau, neurodegeneration, microglia and systemic inflammation in governing the etiology and progression of human AD. This proposal involves a series of principled statistical and mathematical model-based tests that will uncover these relationships, for the first time, directly in patients. This proposal involves a new prospective longitudinal study of 80 AD spectrum patients acquiring brain MRI and PET imaging of activated microglia using a new generation TSPO ligand called DPA-713 and tau-PET imaging using a relatively novel ligand, MK6240. The same imaging protocol (MRI, DPA-713 and MK6240 PET) will be repeated in a 2-year follow up visit. Next this proposal involves developing and testing a model of microglial inflammation-tau interaction via mathematical models to determine whether regional microglia-mediated neuroinflammation measured by DPA-PET is higher in AD- spectrum patients or in cognitively normal older adults; and whether regional microglial activation is predictable directly from tau. Finally, this proposed research includes testing a network spread model of tau and microglia. Mounting animal data implicate a trans-neuronal transmission mechanism of tau through brain networks. Using a network diffusion model of disease spread, this proposal will further investigate the role of microglia in tau progression directly in humans. Since tau and microglia provide complementary signal about evolving pathology, this proposal will determine whether combining imaging studies that measure both biomarkers will result in a uniquely powerful and predictive test of AD progression. Given the rapidly evolving understanding of the role of microglia and systemic inflammation in dementias, the current proposal is timely, topical and necessary for advancing human dementia research. If successful, it will give the first validated spatiotemporal model of the causal interactions between pathology and neuroinflammation in AD, catalyzing future advances in prognostication and targeted anti-inflammatory therapies.